Steel cylinders have been employed for many years as principal components of plastic extrusion and ejection molding equipment. Typically in such equipment, a metal screw is driven within a steel cylinder to force a fluid plastic material through a die. Because the temperatures and pressures involved in the extrusion processes are quite high, and because of the increased use of abrasive fillers in modern plastics, the cylinders (or barrels as they are commonly called) are subjected to abrasion and corrosion which reduces the useful life of the barrels.
Bimetallic cylinders were developed as a solution to these problems. In a typical bimetallic cylinder, the bore of the barrel is coated on its inner surface with a thin coating of an abrasion and corrosion resistant alloy.
The process for preparing such cylinders includes providing a cylindrical steel barrel which has an inside diameter slightly larger than that desired for the final extrusion or injection molding operation. A measured quantity of a suitable lining alloy, usually in pellet or shot form, is then placed inside the bore of the cylinder. The open ends of the cylinder are closed by welding end plates over them and the loaded cylinder is placed in a furnace and heated above the melting point of the lining alloy but below the melting temperature of the backing steel.
Following a heating period determined for the particular lining alloy, the cylinder is placed on a bed of power driven rollers in a horizontal position and rapidly spun to centrifugally cast the melted lining alloy over the cylindrical bore of the barrel. The cylinder is cooled below the melting temperature of the lining alloy during spinning and is then placed within a bed of insulating material for a controlled cooling period to prevent cracking of the alloy and to insure a good bond between the lining and the backing steel.
Finally, the cylinder is prepared for use by removing the end caps and finishing the internal bore to the correct diameter and finish tolerances by conventional lathe and hone techniques.
Throughout the evolution of this process, a great deal of research has been directed to the development of appropriate lining alloys and relatively little attention has been focused on the backing steels. The most commonly used backing steel in the prior art is AISI-4140 steel which has the following composition:
______________________________________ Ingredient Weight Percent ______________________________________ Carbon 0.38-0.43 Manganese 0.75-1.00 Silicon 0.20-0.35 Chromium 0.80-1.10 Molybdenum 0.15-0.25 Iron Balance ______________________________________
This material is not well suited as a backing steel because it is difficult to weld and because it has a tendency to build up residual stresses during the heating and cooling steps in the lining process leading to an increased tendency to bend during extrusion and molding. These difficulties with the prior art backing material can lead to reduced cylinder lifetime and expensive straightening procedures or costly replacement of the entire cylinder. Additionally, it is known that AISI-4140 steels should be used in their heat treated condition for fully utilizing the properties of this alloy, and the repeated heating and cooling of bimetallic cylinder backings during preparation and use is subjecting the steel to conditions which are not recommended. However, 4140 steel is readily available.
Additional properties of AISI-4140 steel are listed below:
______________________________________ Tensile strength psi 90,000 Yield strength psi 65,000-70,000 Elongation % in 2" 27-25 Reduction of area % 55-50 Brinell hardness, surface 200 ______________________________________
A steel backing for bimetallic cylinders which does not have the above-noted disadvantages would be a significant advance in this technology.